For students, particularly in Art History, there is an increasing number of resources for images online. Last month the Metropolitan Museum of Art in New York announced that their entire collection of online images would be freely available for everyone to download and use. You can search the collection by artist, by object type, by region and by date. The images are very high quality and would be ideal for use in presentations. The images are not just of paintings like the El Greco that is the main image for this post, but also objects like this 13th century chalice.
We also recommend checking out the Met Museum’s Hielbrunn Timeline of Art History. This has great introductory articles on a wide variety of topics, like printing or Renaissance drawing, to individual artists such as Caravaggio and Veronese. There are also chronologies of art history from every time period and region of the world. These articles can be very helpful for all students interested in art and the social context of artistic movements; the articles link to specific works in the Met collections and also have references and suggestions for further reading.